1. Field of the Invention
The present invention relates to safety monitors and appliances for a wood pulping mill environmental control system. More particularly, the present invention relates to the method of and apparatus for monitoring the combustibility of non-condensible digester blow gas.
2. Description of the Prior Art
The kraft process of delignifying wood chips for the purpose of cellulose fiber liberation releases small quantities of highly volatile turpentine and mercaptan compounds in the form of substantially non-condensible vapors. Although the quantity of such gases produced by the process is extremely small in relation to the useable mass of the wood, nevertheless, such compounds are substantially responsible for the characteristic odor generally associated with pulp mills.
In the interest of reducing this environmental irritation, pulp mills have, in recent years, eliminated the atmospheric venting of all digester blow gases. According to the best of practice, these gases are isolated from the liquid and solid constituents of a digester discharge at the blow tank to be processed separately to, first, recover steam heat value and, secondly, strip the water soluble compounds.
Those gases remaining in the vapor stream following condensation and water stripping are, substantially, the volatile, odoriferous compounds first described. Due to the combustibility thereof, these gases are burned in the firebox of a convenient heating appliance such as a lime kiln or recovery furnace. It is in the final transport of these isolated flammables that significant hazards arise.
Ideally, such gases should be volumetrically concentrated under increased pressure and delivered to the heating appliance in a controlled manner as a fuel. However, the absolute quantity and specific heating value thereof is normally insufficient to justify the capital cost of compressors and accessory equipment required. Nevertheless, the total vapor pressure of the gas must be sufficient to permit a flow transfer into a normally negative pressure firebox. This circumstance is resolved by discharging the odor control draft induction fan required by the non-condensible gas stream stripper unit directly into the suction flow of the heating appliance primary draft fan. However, the odor control fan includes a fresh air source on the draft side thereof which mixes with the noncondensible gas flow stream. Normally, the resulting mixture is too fuel-lean to be combustible. It is only when the noncondensible gas stream is particularly rich that a combustible mixture may be created which fills the duct work between the fresh draft air confluence and the non-condensible gas stream. If this combustible mixture is subjected to a convenient ignition source, considerable equipment destruction will occur.
Although numerous electrolytic sensory devices are available to continuously measure the combustibility of the critical gas flow stream, long term reliability of such devices in the corrosive atmosphere of a pulp mill is less than satisfactory.
It is the object of the present invention, therefore, to teach a method of reliability monitoring the combustibility of blow gases prior to the firebox of a major heating appliance.
Another object of the invention is to provide fabrication details of a reliable apparatus for monitoring the combustibility of a digester flow gas disposal system.